Using
the Pointillize filter on numbers traced out by shifts in one color, we
can create images such as the one below where people with normal vision
can see the numbers "52", and where people who have red-green
color-blindness can only see the "5".

Now
if we create an image where the red spectrum varies more than the luminosity,
and it is the luminosity that traces out a number, then we have an image
where only people who are red-green colorblind can see the hidden number.
This is because people with normal color-vision are thrown off by the red,
which the red-green colorblind people can't see. In the image below create
using this method, people with red-green color-blindness will see a clearly
defined circle.

We
can even be a little more devious and create an image where people with
normal color-vision will see one thing and people with red-green color-blindness
will see another. But this is slightly more difficult and the result is
only moderately satisfying. Below is an image where people with red-green
color-blindness can see a circle and people with normal color-vision can
see the letter "s". However, there are probably people who have
mild red-green color-blindness who can see both.